The present invention relates to phase separation control of multiphase plural inverter systems and, more particularly, to such controls wherein the phases of the second inverter output signals are offset from the phases of the first inverter output signals in order to further suppress unwanted harmonics.
Copending U.S. patent application Ser. No. 07/745,425, filed on Aug. 15, 1991 discloses an inverter control system wherein a first inverter is controlled by a difference in DC content between each phase of an output AC and neutral and a second inverter is controlled based upon the difference in DC content between corresponding first and second inverter output phases. Copending U.S. patent application Ser. No. 454,434 filed on Dec. 20, 1989, show that by controlling the phase relationship between first and second inverters, the magnitude of the output AC may be regulated. Furthermore, U.S. Pat. No. 4,875,149 discloses a system for separating the phases of an inverter. The present invention recognizes that further harmonics may be suppressed by controlling the output of a second inverter such that each output phase from the second inverter is offset from its corresponding phase produced by a first inverter.
Power inverters for converting DC power into AC power are typically of the polyphase type wherein a plurality N of AC phase outputs are produced. During operation of the inverter, an unbalanced load condition can arise wherein the loads on the phases are unequal. Typically, a polyphase filter is coupled to the inverter output which introduces phase shifts as a function of load. For such an inverter and filter, an unbalanced load condition can cause unequal phase shifts among the phases, resulting in phase errors. These phase shifts are undesirable inasmuch as most polyphase loads require polyphase power at precise angular displacements.
Such phase shifts are even more objectionable where the inverter is a part of a variable speed, constant frequency (VSCF) generating system which is connected in parallel with another VSCF system to one or more loads. In this case, the displacement between corresponding parallel-connected phases of the VSCF system determines the real load power share therebetween. If the displacements of the output phases of both systems are not controlled precisely, a highly undesirable load share condition may arise, in turn resulting in damage or destruction of one or more components.
Moreover, as discussed in our copending patent application Ser. No. 07/745,425, filed on Aug. 15, 1990, advantages can be obtained from using plural inverters in order to share power supplied to the loads. That copending application shows a way in which the first and second controllers can be controlled in order to minimize DC content. The present invention can be used in addition to or as an alternative to the second control disclosed in copending patent application Ser. No. 07/745,425, filed on Aug. 15, 1991. In the present invention, the outputs of the second inverter are phase shifted from the outputs of the first inverter in order to reduce harmonics in the power supplied to the loads.